The present invention relates to a low voltage protection circuit which can be used for battery back-up.
The back-up battery is used as a power source when the power source of the system which uses a volatile, memory together with a microprocessor turns off. It is well known that the usual back-up battery has difficulty returning to normal voltage even by recharging when it is overdischarged below the required voltage, namely, termination voltage, thus shortening its life. Therefore, it is necessary for the back-up battery to be made not to discharge below termination voltage when it discharges, namely, when it is used as a power source for a load.
The circuit heretofore in use to prevent a back-up battery from overdischarging below termination voltage is formed as illustrated in FIG. 1. That is, a back-up battery is connected to terminals 1, 2 by polarity as illustrated. When the battery is charged with normal voltage, the values of resistances R2, R3 are so fixed as to enable the voltage which is inputted to the voltage on both ends of resistance R3 divided by resistances R2, R3, namely, to the reversing terminal of comparator 3 to become higher than the standard voltage by zener diode ZD1, namely, the termination voltage of the battery and the output of the comparator 3 is thereby made to output "low". Accordingly, a relay RY is prevented from being driven by turning a transistor Q1 off and a switch SW1 interlocked with the relay RY turns "on" and a switch SW2 turns "off". Thus, the voltage of the battery is supplied to the load connected to terminals 4, 5. However, when the voltage of the battery reaches termination voltage, the voltage which is inputted to the reversing terminals (-) of the comparator 3 becomes lower than the standard voltage of non-reversing terminal (+). So the output of the comparator 3 becomes "high" and the transistor Q1 turns "on". Then, the switch SW1 turns "off" by the drive of relay RY and the power source of the battery is thereby cut off from the load. At the same time, the output of the comparator 3 is made "low" and the transistor Q1 "off" by turning the switch SW2 on.
However, such a low voltage protection circuit as described hereinbefore is a mechanical-type low voltage protection circuit which employs a relay. It involves a problem in that the battery operates even in a condition where it is not charged sufficiently because it supplies power source voltage to the load with termination voltage as the starting point.